SU Carbs Bogging While Accelerating at Low RPM's

[Nismo49]

Hey, I have been having some trouble my SU Carbs Bogging out. Help!

My 1973 240Z with Round Top SU's is bogging out and sometimes stalling on acceleration with low engine RPM's. Pulling the choke on helps to aleviate this condition while accelerating up to highway speed. The engine runs great driving at higher RPMs (ie) 60 - 80 mph....or higher when the road is clear....it runs up to 120 mph...tail wind on nice level highway. When I slow down to make a turn and try to accelerate after making the turn, the engines sputters and boggs untill I pull on the choke and then it will hook up and accelerate smoothly. The engine sometimes back fires, boggs, coughs in this situation.

The other strange condition is the engine will run reasonable smooth with the choke fully on while at highway speed..one would think that it should run rich and sputter and bogg out..other than stinky exhaust and some smoke, it runs fairly smooth.

I have installed a new gas line filter, used some carb cleaner and this bogging issue still exists as before. The engine also is hard to accelerate from a standing stop...it quite often stalls when taking off from a stop. Pulling on the choke most often aleviates this problem.

Another point of interest is that starting cold takes a lot of cranking to get it to fire up. My 240Z is an automatic...I have heard that the jets are different for the Automatics. Holding the accelerator pedal to the floor helps..and the choke fully on....it will take up to a minute of spinning to get the engine to fire the first time. Usually takes another 2-3 times on the starter before the engine will remain running with out stalling. The engine takes about 3-5 minutes to warm up and smooth out at idle.

Starting hot usually is perfect..it will fire on less than one turn over if the engine has been shut off for up to an hour.

PS: Previous owner had a complete new OEM SU Carb Kits installed less than 4 years ago. 3000 Miles since carb kits installed.

I am looking for some advice on where to start making adjustments or service recommendations. Thanks

Dave

NISMO49

1973 240Z - 98% Original Documented California Car

110 Red - Original Paint

Automatic Transmission

42,000 Original Miles

[Zs-ondabrain]

Try this link out, it has everything SU related that you EVER want to know.

http://www.teglerizer.com/sucarbs/index.html

Dave.

[DaveN]

David,

Your symptoms -- running lean, worse at low RPM -- suggest an air leak into the manifold. It's time to start troubleshooting things like the throttle shafts, vacuum advance, brake booster, and BCDD actuator diaphrams.

Re SU needles -- The 3 screw round tops used on 72's came with N-58 needles in automatics and slightly richer N54's in 4 sp cars. All 4 screw SU's, used in 70 and 71, came with N-27's.

[Bruce Palmer]

David,

Have you tried adjusting the nozzles down in increments to richen them up?

Secondly I'd shoot carb cleaner at the throttle shaft openings and all the mounting flanges toward the head to search out any vacuum leaks.

Our JUST SUs video does a fair country job of walking one through all this....

[beandip]

What wt oil are you using in the dampeners. To thin of oil can cause the engine to stumble at first throttle opening at low rpm. Try some 10w30 and better yet 20wt available at the local cycle shop. ( fork oil) Cold start up. Depressing the throttle actually has vary little effect when starting the engine . Actually if the mixture is set correctly , just adding choke should be all that is necessary. As Bruce stated, sounds like a lean condition also. Check the vacuum advance also. Gary

[Nismo49]

Hi Dave,

I checked out the website and found many references that will be helpful. From what I have read so far...Ihave a lean condition or an air leak as others have mentioned. Really appreciate your references. Thanks

Dave

[Nismo49]

Hi Guys,

I think that the previous owner told that he was using ATF in the dampers. The article that I read from Dave's website resource suggested 20W-50 for worn carbs as a third choice after ATF and 10w-30. I have some 20W-50 oil on hand ...will change it out and try that first before I get into changing the settings. Its about trial and error and learning....I have little experience in setting SU carbs.

Thank You to everyone that responded..really appreciate the sugestions and recommendations. Will keep eveyone posted on my progress.

Dave

Nismo49

What wt oil are you using in the dampeners. To thin of oil can cause the engine to stumble at first throttle opening at low rpm. Try some 10w30 and better yet 20wt available at the local cycle shop. ( fork oil) Cold start up. Depressing the throttle actually has vary little effect when starting the engine . Actually if the mixture is set correctly , just adding choke should be all that is necessary. As Bruce stated, sounds like a lean condition also. Check the vacuum advance also. Gary